An electrorheological fluid is known as a fluid which is obtained by dispersing disperse-phase particles in an electrically insulating dispersion medium, and which has a rheological characteristic that changes from the Newtonian viscosity to the Bingham viscosity under an externally applied electric field. It is also generally known that an electrorheological fluid composition shows a so-called Winslow's effect that the viscosity thereof greatly increases and a large shear stress is induced under an externally applied electric field.
Since the Winslow's effect has a characteristic that it shows a shear stress which quickly changes in response to an externally applied electric field, the electrorheological fluid can be applied to a variety of driving devices such as a clutch, a brake, an engine mount, a damper, a valve, a shock absorber, an actuator, an inkjet using an electrorheological fluid, etc.
Examples of conventional electrorheological fluids in which a carbon material is used as a disperse-phase include: fluids using insulating (dielectric) carbon materials such as meso-phase carbon (Japanese Unexamined Patent Publication No. 45196/1992 (Tokukaihei 4-45196), carbonaceous powders (Japanese Laid-Open Patent Publication No. 169025/1990 (Tokukaihei 2-169025), Japanese Laid-Open Patent Publication No. 47896/1991 (Tokukaihei 3-47896), Japanese Laid-Open Patent Publication No. 247696/1991 (Tokukaihei 3-247696), Japanese Laid-Open Patent Publication No. 247698/1991 (Tokukaihei 3-247698), Japanese Laid-Open Patent Publication No. 279206/1991 (Tokukaihei 3-279206), Japanese Laid-Open Patent Publication No. 211499/1992 (Tokukaihei 4-211499), and Japanese Laid-Open Patent Publication No. 348192/1992 (Tokukaihei 4-348192); fluids using insulating (dielectric) composite materials in which an electrically conductive carbon material is dispersed in an insulating matrix so as to apply an insulating (dielectric) property such as a composite material in which carbon black is dispersed in a resin (Japanese Laid-Open Patent Publication No. 236291 (Tokukaihei 1-236291). However, the above-listed electrorheological fluids present the problem of poor dispersion stability due to the agglomeration and settlement of the disperse-phase particles in the dispersion medium.
Electrorheological fluids containing dielectric materials obtained by coating the surface of an electrically conductive particle with the electrically insulating thin film have been proposed. These examples include fluids using electrically conductive particles with a surface coated with the electrically insulating thin film (Japanese Laid-Open Patent Publication No. 6093/1989 (Tokukaisho 64-6093), and fluids using carbon particles having a surface thereof coated with the electrically insulating thin film (Japanese Laid-Open Patent Publication No. 169025/1990 (Tokukaihei 2-169025). Regarding the above-mentioned fluids, an application of these particles of several microns or above was examined, and discovered that the electrorheological in which these particles were used as the disperse-phase had poor dispersion stability. Moreover, if the electrically conductive particles or carbon particles are below sub micron, an interaction between the particles is strong. Therefore, when the coating methods such as the micro capsule method or the surface treatment using a silane coupling agent are applied to the less-than sub micron particles, the aggregation between particles cannot be prevented. Therefore, the problem is present in that the desirable dielectric materials for disperse-phase cannot be obtained. Especially when the electrically conductive particles or carbon fine particles are the carbon black of less-than sub micron diameter, an interaction between the carbon black particles is extremely strong, and a desirable dielectric material for the disperse-phase cannot be obtained.
As a method for simultaneously providing an insulating property and dispersion stability to electrically conductive particles, a technique for reacting a reactive silicone oil with a treated particle which is obtained by treating the surface of an inorganic electrically conductive particle with a silane coupling agent has been proposed (Japanese Laid-Open Patent Publication No. 120196/1992 (Tokukaihei 4-120196). According to this disclosure, however, since the processing using the silane coupling agent becomes complicated, the manufacturing cost increases. Moreover, since the coupling reaction is a condensation reaction which produces water, the problem is present that the resulting water lowers the insulating properties of the electrorheological fluid. Moreover, when the electrically conductive particles are carbon black of less than sub-micron diameter, an interaction between carbon black particles is very strong. Therefore, when processing with the silane coupling agent, the aggregation of the particles occurs, and even if the resultant substance is reacted with the reactive silicone, desirable dielectric particles for the electrorheological fluid cannot be achieved.
Examples of electrorheological fluids which show excellent dispersion stability include: fluids using liquid crystal compounds (see Japanese Laid-Open Patent Publication No. 191511/1992 (Tokukaihei 4-191511), Japanese Laid-Open Patent Publication No. 266997/1992 (Tokukaihei 4-266997), Japanese Laid-Open Patent Publication No. 337389/1992 (Tokukaihei 4-337389), Japanese Laid-Open Patent Publication No. 348194 (4-348194)), and a uniform electrorheological fluid such as fluids using dielectric polymer (YURNS research report, 2, 58 (1990), etc.). However, the above listed electrorheological fluids present the problem of high viscosity and poor fluidity in an absence of the electric field. Especially, when the liquid crystal compound is used, a high cost is required.